Radio telephones, commonly called cellular (or “cell”) phones, have become ubiquitous in recent years. Formerly the domain of the wealthy, or those in specialized professions for whom the great expense then associated with them was justified, radio telephones are now used by a majority of the population in this country and in many other regions around the world. Considerable leaps in technology have contributed significantly to this evolution. These advances have not only made radio telephone service available to many subscribers at a reasonable price, but they have also permitted great increases in the capacity of the communication networks providing the service.
The cell phone is so called because it is designed to operate within a cellular network. Such a network has infrastructure that switches and routes calls to and from network subscribers who are using portable radio devices. Rather than having one or two antennas to handle all of this radio traffic, however, the cellular network is divided into a great many smaller areas, or “cells”, each having an antenna of their own. A cellular wireless system has several advantages over a central antenna system. As the cells are much smaller than the large geographic area covered by a central antenna, transmitters do not need as much power. This is particularly important where the transmitter is housed in a small device such as a cell phone. In addition, the use of low-power transmitters means that although the number of them operating in any one cell is still limited, the cells are small enough that a great many may operate in an area the size of a major city. The mobile stations do not transmit with enough power to interfere with others operating in other cells, or at least those cells that are not adjoining. In some networks, this enables frequency reuse, that is, the same communication frequencies can be used in non-adjacent cells at the same time without interference. This permits the addition of a larger number of network subscribers. In other systems, codes used for privacy or signal processing may be reused in a similar manner.
At this point, it should also be noted that as the terms for radio telephones, such as “cellular (or cell) phone” and “mobile phone” are often used interchangeably, they will be treated as equivalent herein. Both, however, are a sub-group of a larger family of devices that also includes, for example, certain computers and personal digital assistants (PDAs) that are also capable of wireless radio communication in a radio network. This family of devices will for convenience be referred to as “mobile stations” (regardless of whether a particular device is actually moved about in normal operation).
In addition to the cellular architecture itself, certain multiple access schemes may also be employed to increase the number of mobile stations that may operate at the same time in a given area. In frequency-division multiple access (FDMA), the available transmission bandwidth is divided into a number of channels, each for use by a different caller (or for a different non-traffic use). Time-division multiple access (TDMA) improves upon the FDMA scheme by dividing each frequency channel into time slots. Any given call is assigned one or more of these time slots on which to send information. More than one voice caller may therefore use each frequency channel. Code-division multiple access (CDMA) operates by spreading and encoding transmissions. By encoding each transmission in a different way, each receiver (i.e. mobile station) decodes only information intended for it and ignores other transmissions.
The number of CDMA mobile stations that can operate in a given area is therefore limited by the number of encoding sequences available, rather than the number of frequency bands. The operation of a CDMA network is normally performed in accordance with a protocol referred to as IS-95 (interim standard-95) or, increasingly, according to its third generation (3G) successors, such as those sometimes referred to as 1xEV-DO and 1xEV-DV, the latter of which provides for the transport of both data and voice information.
A wireless network using any of these schemes employs a certain basic structure such as the one illustrated in FIG. 1. FIG. 1 is a simplified block diagram illustrating selected components of a wireless transmission system 100. Wireless transmission system 100 includes a transmit side 105 and a receive side 155. This illustration implies that the two sides are located in different terminals that are attempting to communicate with each other, although typically a communication terminal will include both transmit and receive functions.
The information to be transmitted, which may be voice or data information, is first provided to an encoder 110 to be encoded into digital form. Note that the terms ‘data’ and ‘information’ may be used interchangeably herein. No formal distinction is thereby intended unless it is specifically stated or apparent from the context. The encoded information is then mapped to symbols in a modulator 120 and provided to transmitter 130, where it is modulated onto a carrier wave and amplified for transmission via radio channel 150 through antenna 140.
The receiver 170 receives the transmitted radio frequency (RF) signal x through antenna 160. The received signal y is processed by the receiver 170 provides and the result {circumflex over (d)} to a demodulator 180, which recovers the encoded sequence û (as well as it is able) taking into account the characteristics h of channel 150. This encoded sequence û is provided to a decoder 190 for replication of the originally transmitted information. As should be apparent, the goal of any such communication system is the faithful reproduction of this information.
There are a number of obstacles, however, to reliable and effective transmission of information over the air interface. One of the most significant is multipath fading. Transmitted radio signals, generally speaking, spread out as they propagate, and different portions of the signal may reflect off or be otherwise impeded by the various objects each portion encounters. The result is that the different portions of same signal take different paths to the receiver and therefore arrive at slightly different times. These different portions may then interfere with each other and cause fading.
One manner of addressing this challenge is through the use of transmission diversity, for example time diversity or space diversity. Time diversity involves introducing time-delayed redundancy into the transmitted data and, where the fading is time variant, allows the receiver to more accurately recover the transmitted information. Spatial diversity may also be used. In spatial diversity more than one transmission antenna is used, the antennas being spaced apart at a distance selected to provide a desired level of correlation between the data transmitted by each of the antennas. A combination of these two types of transmit diversity may be referred to as space-time transmit diversity (STTD).
The present invention is a receiver, a system, and a method for utilizing STTD transmitted signals and is of particular advantage when applied to a third-generation CDMA network, for example one operating according to the 1xEV-DV protocol.